c# - 附加属性回调不适用于 wpf VS2012 中的默认值

Question

我正在尝试为组合框编写一个简单的附加属性，它通过回调更改组合框的数据源我的问题是回调函数对默认值不起作用，这是我的代码

附加属性类

namespace WpfApplication2
{
    public enum Types { Employee, Position, Task }
    public class ComboBoxAttached:DependencyObject
    {
        public static readonly DependencyProperty TypeOfProperty =
            DependencyProperty.RegisterAttached(
                "TypeOf", 
                typeof(Types), 
                typeof(ComboBoxAttached), 
                new PropertyMetadata(Types.Position, OnTypesChanged));  

        public static void SetTypeOf(DependencyObject d, Types use)
        {
            d.SetValue(TypeOfProperty, use);
        }

        private static void OnTypesChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ComboBox cb = d as ComboBox;

            // MessageBox.Show(((Types)e.NewValue).ToString());
            switch ((Types) e.NewValue)
            {
                case Types.Employee:
                    cb.ItemsSource = SourceData.Employee;
                    break;
                case Types.Position:
                    cb.ItemsSource = SourceData.Position;
                    break;
                case Types.Task:
                    cb.ItemsSource = SourceData.Task;
                    break;
            }
        }
    }
 }

数据源的类

namespace WpfApplication2
{
    public static class SourceData
    {
        public static List<string> Employee=new List<string>(){ "Manager","HR","CEO","CFO"};
        public static List<string> Position = new List<string>() { "Right", "Left", "Forward", "Backward" };
        public static List<string> Task = new List<string>() { "Assessment", "Measurement", "Consult", "Other" };  
    }
}

xaml

<ComboBox MyProp:ComboBoxAttached.TypeOf="Position" Margin="5" />

Answer 1

只有当属性实际改变时才会调用回调。最初它有它的默认值，并且根据设计不会调用回调。所以你要么需要手动调用它，要么你可以使用 Types? 类型的属性(让它可以为空)让默认值为 null:

public static readonly DependencyProperty TypeOfProperty =
            DependencyProperty.RegisterAttached(
                "TypeOf", 
                typeof(Types?), 
                typeof(ComboBoxAttached), 
                new PropertyMetadata(null, OnTypesChanged));